The present invention relates to "hybrid" or compound vacuum pumps which have two or more sections of different operational mode for improving the operating range of pressures and throughput; and more particularly, to oil free (dry) compound vacuum pumps.
A screw pump comprising two externally threaded or vaned rotors mounted in a pump body and adapted for counter-rotation in the body with intermeshing of the rotor threads is well known. Close tolerances between the rotor threads at the points of intermeshing and with the internal surfaces of the pump body causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface of the pump body and thereby urged through the pump as the rotors rotate.
Such screw pumps are potentially attractive because they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the pump inlet down to atmospheric pressure at the pump outlet.
Screw pumps are generally designed with each screw rotor being of cylindrical form overall, with the screw thread tip cross section being substantially constant along the length of the rotor. This has a disadvantage in vacuum pumps in particular that no volumetric compression is generated in use of the pump along the length of the rotor, thereby detrimentally affecting the pump's power consumption.
A further disadvantage commonly encountered with screw pumps in that they can suffer from low pumping speeds at relatively low inlet pressures, for example of the order of 50 mbar or less.
The present invention is concerned with overcoming such disadvantages and to provide a screw pump with improved power consumption coupled with improved inlet speeds.